Measurement of OH concentration profiles by laser diagnostics and modeling in high-pressure counterflow premixed methane/air and biogas/air flames

Detailed kinetic mechanisms validated under atmospheric or low-pressure flame conditions cannot generally be directly extrapolated toward high pressure, due to the lack of experimental data or to uncertainties concerning the rate constants of elementary reactions. It is thus necessary to complete the experimental database and to extend the validation domain of combustion mechanisms at high pressure. In this work, OH concentration profiles were measured in high-pressure methane/air and biogas/air laminar premixed counterflow flames by linear laser-induced fluorescence at different equivalence ratios (0.7–1.2) and pressures (0.1–0.7 MPa). Each flame was calibrated in absolute OH concentration by a combination of laser absorption and planar laser-induced fluorescence measurements. Experimental results were compared with simulations using the OPPDIF code and three detailed kinetic mechanisms: two versions of the Gas Research Institute Mechanism, GRI-Mech 2.11 and GRI-Mech 3.0, and the recently updated GDFkin®3.0_NCN mechanisms. Results show that the flame front positions are very well predicted by modeling with the three mechanisms for lean and stoichiometric CH4/air flames and stoichiometric CH4/CO2/air flames. However, discrepancies appear at a higher equivalence ratio (Φ = 1.2) for the CH4/air flames and at a lower equivalence ratio (Φ = 0.7) for the CH4/CO2/air flames. OH mole fractions are quantitatively well predicted at high pressure in all cases, while systematic overestimation by modeling is observed at atmospheric pressure. A kinetic analysis of the results is also presented.